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Even U.S. Fish & Wildlife Service cannot contact Palmer, asks him to call them

Walter Palmer kills a beloved lion in Africa

Police beef up patrols in Palmer's neighborhood

(CNN)Dr. Walter Palmer is nowhere to be found.

The Minnesota dentist has gone underground in the onslaught of criticism after he killed a prized African lion named Cecil.

It probably shouldn't come as a surprise; an angry horde of Cecil supporters is calling for his head to be mounted on a wall.

Meanwhile, the U.S. Fish and Wildlife Service is investigating the circumstances surrounding the lion's killing.

"At this point in time, however, multiple efforts to contact Dr. Walter Palmer have been unsuccessful. We ask that Dr. Palmer or his representative contact us immediately," said Edward Grace, the service's deputy chief of law enforcement.

In a letter dated July 28 to his patients, Palmer said the hunting controversy "has disrupted our business and our ability to see our patients."

"For that disruption, I apologize profoundly for this inconvenience and promise you that we will do our best to resume normal operations as soon as possible," he said in the letter obtained by CNN affiliate WCCO.

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"I had no idea that the lion I took was a known, local favorite, was collared and part of a study until the end of the hunt," Palmer said Tuesday in a statement. "I relied on the expertise of my local professional guides to ensure a legal hunt."

Two Zimbabweans have been charged in the case and officials in the African nation say they want to talk to Palmer. The undercover dentist has indicated that he'll cooperate, although he said in a statement that he had yet to be contacted by anyone about the investigation.

"I think like most people in the world, we are outraged at what happened to this poor lion," Harald Braun, Germany's ambassador to the United Nations, told reporters Thursday.

Braun spoke minutes after the U.N. General Assembly passed a resolution calling on all countries to crack down on illegal wildlife trafficking and poaching. The vote came after two years of work on the resolution and was not tied to Cecil's death.

Cecil's killing doesn't appear to be the first time Palmer has gotten into trouble while hunting.

A man by the same name and age, and from the same town, illegally killed a black bear in Wisconsin several years ago, according to court documents.

That individual pleaded guilty to making false statements knowingly to the U.S. Fish and Wildlife Service, and was sentenced to one year on probation and ordered to pay a fine of nearly $3,000, records show.

A New York Times article in 2009 that profiled Palmer and his hunting methods said he had served a year of probation over the false statements case.

The Times article detailed Palmer's skill and enthusiasm for using archery rather than firearms to slay animals.

He is "said to be capable of skewering a playing card from 100 yards with his compound bow," it said, recounting his killing of a large elk with an arrow in Northern California.

Above the menagerie of stuffed animals at the door, posters now cover the front facade of the practice.

One sign asked, "Dr. Palmer, why did you kill Cecil?" Another said, "Rot in hell." A third employed the hashtag #catlivesmatter.

The vitriol for Palmer even flowed from the governor's mansion.

"I'm just so disgusted with that man," said Gov. Mark Dayton. "Shoot any lion but lure a lion like that out of the preserve and shoot him, how could anybody think that's sport? Just appalling."

And the mood in the area of Palmer's home, in Eden Prairie, was dicey enough for police to issue a statement saying they were protecting the area -- although Palmer could fend for himself.

"Because of the increased traffic in the neighborhood of Walter Palmer's residence, the Eden Prairie Police Department is monitoring the neighborhood to ensure the safety and security of the residents and their property," the statement read. "The Eden Prairie Police Department is not providing personal protection for Mr. Palmer."

With the storm of criticism continuing to brew, it may be quite some time before the hunter, now hunted, feels like it's safe to come out.